This invention relates to a tank structure for a hot-water supply unit, and more particularly to a tank structure for a hot-water supply unit suitable for use in a hot-water supply system or a hot-water space heating system which is simply installed and keeps a temperature of hot water stable.
In general, a conventional hot-water supply unit proposed for simple installation is constructed into a pressure direct-application structure wherein a metal piping is wound around a combustion chamber to constitute a heat exchange section, to thereby feed tap water directly to the metal piping for heating. Such construction of the conventional hot-water supply unit permits tap water under a high pressure to be directly supplied thereto, to thereby highly simplify a piping structure, because the heat exchange section is constituted by the metal piping exhibiting highly increased pressure resistant characteristics. Also, it has another advantage of extensively facilitating installation of the unit.
Thermal transfer to the metal piping in the conventional hot-water supply unit is carried out by heating a metal wall of a combustion chamber and then transferring heat of the combustion chamber to the metal piping to heat it, resulting in thermal efficiency being highly deteriorated. It would be considered to increase a length of the metal piping, to thereby increase a heat transfer area of the piping. However, this causes water fed to the piping to be boiled at portions of the piping locally excessively heated, resulting in whole heat efficiency of the piping being conversely deteriorated. Thus, the conventional hot-water supply unit substantially fails to increase heat efficiency. Also, it renders temperature control of hot water highly troublesome because tap water fed to the metal piping is heated during passage thereof through the metal piping.
Another hot-water supply unit is also proposed. The unit proposed includes a tank body of a double-barrel structure constituted of an inner barrel arranged around a combustion chamber and an outer barrel arranged outside the inner barrel. Water is fed to the tank body and heated therein, followed by discharge of the heated water through a hot-water discharge pipe. In order to increase heat exchange efficiency of the tank body, the inner barrel has a height smaller than that of the outer barrel and a plurality of connection pipes are arranged between an inner barrel top plate and an outer barrel top plate to heat water outside the connection pipes. However, the tank body causes a configuration thereof to be complicated with an increase in heat exchange area thereof, leading to a deterioration in pressure-resistant characteristics thereof. Thus, the unit fails to feed tap water under a high pressure to the tank body, so that it is required to arrange a safety mechanism such as a pressure reducing valve, a safety valve or the like in the piping to prevent breakage of the tank body.
The hot-water supply unit permits the amount of water stored in the tank body of the double-barrel structure to be increased and a heat transfer area to be readily increased with an increase in length of the connection tubes and the number thereof, resulting in highly increasing heat exchange efficiency. Also, an increase in the amount of water stored in the tank body ensures constant and stable feeding of hot water and minimizes a variation in temperature of hot water in use, when a temperature in the tank body is kept at a predetermined level by heating.
However, the hot-water supply unit of the double-barrel structure is restricted to operation at a water pressure as low as at most about 5 kgf/m.sup.2 because the tank body is complicated in structure. Thus, it fails in operation at a water pressure as high as 21 kgf/m.sup.2 as applied to the above-described unit of the metal piping type which is connected directly to a water service pipe. Thus, it necessarily requires arrangement of a pressure reducing valve for reducing a water pressure to a level of 1 kgf/m.sup.2 or a safety device for relieving an excessive pressure applied to the tank body. Also, it requires arrangement of a drainage pipe for drainage when an excessive pressure is applied thereto. Such situation causes a piping around the tank body to be highly complicated.
It would considered that an increase in thickness of a plate material for the tank body leads to an increase in pressure resistance thereof even when the tank body is complicated in structure. However, an equipment for bending or curving a plate material increased in thickness into components of the tank body must be constructed in a specific manner, leading to an increase in cost and lack of universality, so that an investment in the equipment is wasted. Thus, actually, the tank body thus increased in thickness has not been available. Also, such an increase in thickness of the tank body causes an increase in weight of the tank body, resulting in installation of the unit being highly troublesome. Thus, the hot-water supply unit including the tank body of the double-barrel structure has not been put to practice from the viewpoints of both manufacturing and installation.